This invention relates generally to crystal pulling machines and more particularly to a method and apparatus for adjusting the position of a pull wire of a crystal pulling machine.
The substantial majority of monocrystalline silicon used to make silicon wafers for the microelectronics industry is produced by crystal pulling machines employing the Czochralski method. Briefly described, the Czochralski method involves melting chunks of high-purity polycrystalline silicon in a quartz crucible located in a specifically designed furnace to form a silicon melt. A relatively small seed crystal is mounted above the crucible on the lower end of a pull wire hanging from a crystal lifting mechanism for raising and lowering the seed crystal. The seed crystal is lowered into contact with the molten silicon in the crucible. When the seed begins to melt, it is slowly withdrawn from the molten silicon and starts to grow, drawing silicon from the melt. During the growing process, the crucible is rotated in one direction and the crystal lifting mechanism, wire, seed, and crystal are rotated in an opposite direction.
A problem encountered in such crystal pulling machines is pendular motion (i.e., swinging or orbiting) of the wire and crystal during the growing process. Pendular motion may be caused by eccentricity (i.e., misalignment) of the pull wire centerline with respect to the axis of rotation of the crystal lifting mechanism. Such eccentricity may be due in part to imperfections in the concentricity and precision of bearing assemblies used for rotating the crystal lifting mechanism. Pendular motion may also be caused by factors which arise during crystal production, such as asymmetrical crystal growth and/or external disturbances.
Pendular motion of the pull wire and crystal during crystal production imparts bending load stresses to the neck of the crystal. If the bending stresses are too high, the neck may break and the crystal may fall into the crucible, thereby ruining the crystal. Even smaller pendular motions (i.e., pendular motions which are insufficient to break the neck of the crystal) are detrimental to crystal production. Such pendular motions may cause crystal distortion or impede the ability of diameter sensors and controllers to detect and control crystal diameter. They may also result in dislocations in or otherwise reduce the quality of the produced crystal.